1. Field of the Invention
The invention concerns a magnetic resonance tomography apparatus with a local coil that is fashioned to receive a magnetic resonance signal, and with a detector system that is fashioned to detect a position of the local coil. Moreover, the invention concerns a method to detect a position of a local coil in a magnetic resonance tomography device, the local coil being fashioned to receive a magnetic resonance signal.
2. Description of the Prior Art
Magnetic resonance tomography is a technique in widespread use for the acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or a body part of the patient that is to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field that is generated by a basic field magnet of the magnet system of the magnetic resonance tomography device. Rapidly switched gradient fields for spatial coding that are generated by gradient coils of the magnet system are and overlaid on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, RF pulses of a defined field strength are radiated into the examination volume (in which the examination subject is located) with a radio-frequency antenna. The nuclear spins of the atoms in the examination subject are excited by means of these RF pulses such that they are deflected out of their equilibrium state (which runs parallel to the basic magnetic field) by an amount known as an “excitation flip angle”. The nuclear spins then process around the direction of the basic magnetic field. The magnetic resonance signals that are thereby generated are acquired by radio-frequency reception antennas. The reception antennas can either be the same antennas with which the radio-frequency pulses are also radiated or separate reception antennas. A better signal-to-noise ratio is obtained with the use of separate reception antennas. To further improve the signal-to-noise ratio of high resolution images, antenna systems known as high field systems are used that are operated at a basic magnetic field of 3 Tesla. A magnetic resonance tomography device (MRT device) suitable for the magnetic resonance data acquisition as described has a stationary magnet system that embodies the coils necessary to generate the different fields.
Typically it is necessary that more reception antennas should be connected to a receiver system to receive coil signals of the reception antennas than there are receivers provided. This is achieved with the use of a switching matrix that is connected between the receiver system and the reception antennas. The switching matrix relays to the receiver system only the receiver signals that originate from those reception antennas that should actually be active. This procedure is useful because the magnet system has a limited homogeneity volume only in a region of its isocenter, and this homogeneity volume is sometimes relatively short (for example 20 to 40 cm) in relation to the length of the stationary magnet system and the length of a person being examined (who can be covered by a number of reception antennas). The field of view in which respective image data are acquired should lie within this homogeneity volume. It is therefore the goal that signals only from those reception antennas that are located in the homogeneity volume should be read out. However, for this purpose the positions of the individual reception antennas must be determined sufficiently precisely, and in particular it must be decided whether they are positioned within the homogeneity volume or not. The separate reception antennas—also called local coils and abbreviated in the following as coils—can be divided into two classes, namely into stationary coils and non-stationary coils.
Due to their mechanical design, the stationary coils can only be attached to the patient table at specific positions and there act as a head coil or spinal column coil, for example. Cardiac coils or knee coils (for example) that, due to their design, are not necessarily bound to a specific site on the bed or the patient fall in the class of non-stationary coils. These non-stationary coils are usually located on the patient (anterior). Knee coils can be alternately used for the left or right knee. Additional examples of non-stationary coils are abdomen coils, wrist coils, shoulder coils, etc. Such non-stationary coils are typically used in order to achieve an optimally flexible adaptation to the respective anatomy of the patient and play an important role in modern magnetic resonance tomography.
With regard to the identification and localization of stationary coils, it is necessary for the MRT device to precisely identify the type of coil, which is uniquely identified by an electrical code that is transmitted after a connection of the coil to the MRT device, and to unambiguously predetermine the location of the coil, since this is integrated into the patient bed or attached to it. For example, if the patient bed is driven into the stationary magnet system, the position knowledge of the stationary coil is obtained due to the precise position knowledge of the patient bed as well as its speed. By contrast, the non-stationary coils are characterized in that, although which coil type is connected is known to the MRT device via the transmission of a code, any spatial information regarding this coil is absent for the MRT device.
Since, as noted above, reception signals of a coil can only be meaningfully evaluated if it is ensured that this coil is located in the homogeneity volume of the magnet system, an optimally precise determination and knowledge of the position of the coil is indispensable.
The position of the non-stationary coil is typically determined by magnetic resonance measurements (data acquisitions). For this purpose, the components of the MRT device that are used to conduct the magnetic resonance measurement form a detector system to detect the position of the local coil. Since the MRT device has no a priori knowledge about the position of the non-stationary coil on the bed or on the patient, this must initially be “searched for” through a series of such magnetic resonance measurements. The patient bed, with the patient located thereupon with the non-stationary coils, is thereby brought into various z-positions along the longitudinal direction of the magnet system within the magnet system and a magnetic resonance experiment is conducted at the respective z-positions. The position of the non-stationary coils can be derived from the series of the reception signals that is obtained in this way.
However, certain basic conditions must be complied with in order to implement this method for coil position detection. For example, the entire workflow must be organizes with the necessity of conducting the coil position detection being taken into account, because the implementation of the magnetic resonance measurement therefore takes time, and protective measures against interference radiation and signals from the outside must be taken. Typically a magnetic resonance system has a shielding chamber in which the MRT device, the patient bed and the local coils are located. To avoid interference radiation, the door to the shielding chamber must be closed, for example. For this purpose the technician supervising the procedure must leave the room after the patient preparation and close the door after himself, herself or close it from the inside, so that the movement of the bed or of the patient can at least be visually monitored by this person technician during the coil position detection.
Additionally, such coil position determinations with magnetic resonance experiments are problematical if the coil to be localized is located outside or at the edge of the homogeneity volume.